The invention is directed to electric motors and more specifically to magnet and coil assemblies used in electric linear motors.
Electric linear motors are used in various types of electrical equipment. For example, multi-axis positioning stages used in the manufacture of integrated circuits utilize electric linear motors. Conventional linear electric motors generally have a magnet track with pairs of opposing magnets facing each other. Within spaces between the pairs of opposing magnets, an armature moves. The armature has coils of a conductor which are connected to an electrical current. When the electrical current is turned on, the current interacts with the magnetic fields of the magnet pairs to cause a force on the armature. When the armature is attached to a wafer stage, the wafer stage can be made to move in concert with the armature.
For further background information, the reader is directed to the following standard textbooks all of which are incorporated by reference herein in their entirety: Permanent-Magnet DC Linear Motors, A. Basak, Clarendon Press, 1996; Fundamentals of Physics, Second Edition, Extended Version, Revised Printing, David Halliday and Robert Resnick, John Wiley and Sons, 1986; Brushless Permanent-Magnet Motor Design, D. C. Hanselman, McGraw-Hill, 1994; Design of Brushless Permanent-Magnet Motors, J. R. Hendershot, Jr. and T. J. E. Miller, Magna Physics Publishing and Clarendon Press, 1994.
Examples of conventional electric motors having a magnet track with pairs of opposing magnets are shown in U.S. Pat. Nos. 4,151,447; 4,758,750; 4,767,954; and 5,808,381. In U.S. Pat. No. 4,151,447 to von der Heide et al., a stator is disclosed having a pair of ferromagnetic parallel bars, each with a row of magnets extending lengthwise. An armature having a plurality of coils is mounted between the rows of magnets. The armature can travel in the direction of the rows of magnets. In U.S. Pat. No. 4,758,750 to Itagaki et al., another linear motor of the moving-coil type is disclosed. The stationary part in Itagaki et al. includes two opposed magnet paths, each having a plurality of magnets linearly arranged. U.S. Pat. No. 4,767,954 to Phillips also teaches an electric motor having a single coil array with magnets on both sides of the coils. Accordingly, each of the above inventions requires two sets of magnets to move the armature. Consequently, the mass of these linear motors is relatively high.
Another example of a magnet track comprising two rows of magnets is disclosed in U.S. Ser. No. 09/054,766 to Nikon Research Corporation of America which is hereby incorporated by reference. In U.S. Ser. No. 09/054,766, an armature having a plurality of similarly-shaped and overlapping coils is featured. By overlapping a number of coils, the linear motor packs more coils into the limited space between the two rows of magnets. The linear motor has increased efficiency due to the increase in coil density. Its mass, however, remains relatively high because the design requires two rows of magnets. What is needed is a linear motor having less mass.
Various electric linear motors having single magnet arrays are also known. For example, U.S. Reissue Pat. No. 34,674 to Beakley et al. shows an electric motor with a single magnet array. In Beakley et al., coils are aligned along each side of a single magnet array. The design in Beakley et al. further requires a magnetic circuit completion means. Beakley et al. provides that the magnetic circuit completion means is an iron plate positioned along the outside of each coil array. This iron plate increases the mass of the magnet assembly and does not provide a low mass electric linear motor.
U.S. Pat. No. 4,641,065 to Shibuki et al. also discloses a single magnet array type design. In Shibuki et al., a pair of coils are movably disposed along permanent magnets.
U.S. Pat. No. 5,072,144 to Saito et al. shows a stator means having a single permanent magnet array. Moving means are mounted to the stator means series of cores wherein each of the moving means is U-shaped in cross section. Each core further has two leg portions wound with coils to provide electromagnetic force. While the motor contains only a single linear array of magnets, the motor""s mass is increased due to the mass of the cores.
The invention features a low mass electric linear motor having a magnet assembly with a plurality of magnets fixed to a base member. Each magnet has two opposing magnetic surfaces with opposite magnetic poles. The plurality of magnets are attached to the base member such that the opposing magnetic surfaces are aligned and are alternating in magnetic polarity along the base member. A coil assembly is disposed around at least a portion of the magnet assembly. The coil assembly has two walls joined to a header. Each of the walls has a plurality of juxtaposed flat coils and a plurality of bent coils. The bent coils overlap with the flat coils such that a vertical side of each bent coil is positioned within an aperture of a flat coil. In another aspect of the invention, each wall of the coil assembly is enclosed in a cooling canister. Chilled coolant is pumped through the canister thereby removing heat generated by the coil assembly during operation.
An advantage of the present invention is that the magnetic assembly has a relatively low mass. The mass of the magnetic assembly is relatively low since only a single row or array of magnets is used, rather than a dual-magnet track. This is an advantage in, for example, smoothly accelerating a wafer stage of a multi-axis positioning table used in the manufacture of integrated circuits.
Another advantage of the present invention stems from interlocking the coils within the coil assembly. Interlocking the coils increases coil density and since the coils provide electromagnetic force when conducting electricity, an increase in coil density increases the amount of electromagnetic force generated by the coil assembly. Thus, a higher driving force may be achieved using the same size motor.